Our long-term goal is to investigate the in vivo requirements for inducing graft-versus-host disease (GVHD) which will lead to the development of new strategies designed to inhibit GVHD morbidity and mortality. Proinflammatory stimuli upregulate the expression of cell surface receptors and ligands that can deliver positive or negative signals to T cells. While a major effort has been placed on understanding the immunobiology of CD28/B7 supergene family members that inhibit activated T cells, alterations in the intracellular metabolic pathways of APCs and parenchyma! cells can play an important role in immune suppression. Indoleamine 2,3 dioxygenase (IDO) catabolizes the essential amino acid L-tryptophan, causing a state of tryptophan depletion and immune suppression. We have compelling data that IDO knockout (-/-) recipients are hypersusceptible to GVHD-induced tissue injury, weight loss and lethality, indicating a critical role of host IDO in GVHD. We propose to test a series of hypotheses as to the potential cascade of events that leads to IDO upregulation and the mechanism(s) by which the regulation of IDO expression affect GVHD-induced tissue injury. Aim 1: To define the mechanisms by which IDO mediated immune regulation dampens GVHD injury. In aim 1 A, we hypothesize that the kinetics and magnitude of IDO upregulation in host tissues is an important determinant of GVHD organ-specific injury. We hypothesize that negative costimulatory receptors expressed on alloactivated T cells induce IDO via the engagement of their cognate ligands. In aim 1B, we hypothesize that host IDO+ cells act on the initial priming of effector cells within secondary lymphoid organs, to limit GVHD effector cell recruitment into GVHD parenchymal target organs and inhibit GVHD injury via direct protective tissue effects. Aim 2: To devise novel approaches for in vivo GVHD inhibition based upon IDO pathway effects. In aim 2A, we hypothesize that increasing IDO expression in host APCs pre-BMT will provide an immune suppressive environment that will inhibit GVHD effector cell generation post-BMT. We further hypothesize that L-tryptophan catabolites given post-BMT will result in aborted GVHD injury. In aim 2B, we hypothesize that Treg suppression does not require host IDO expression but Tregs and high host IDO expression will be additive in suppressing GVHD. At the conclusion of this application, we will have gained important new insights into the mechanisms by which IDO regulates GVHD destruction and developed novel approaches to inhibit GVHD via the IDO pathway. Public Health Benefits. Our goal is to develop clinically relevant approaches that will facilitate adoptive T cell immunotherapy to treat patients with cancer. The fundamental insights gained from these studies will have broad implications relevant to both cancer therapy and treatment of infectious diseases.